Bandwidth

Bandwidth is the capacity of a connection: it tells you how much data the link can carry in a set period, normally one second. It works like the width of a pipe carrying water: a wider pipe lets more water through at once, and a connection with more bandwidth moves more data in the same amount of time.

Bandwidth is measured in bits per second. Everyday connections are usually described in megabits per second (Mbps), and fast links such as fibre or a school's main internet line in gigabits per second (Gbps). One Gbps is 1000 Mbps. A bit is a single 1 or 0, so Mbps counts millions of bits each second, not millions of bytes. A line advertised at 200 Mbps can carry at most 200 million bits every second.

Why higher bandwidth helps

More bandwidth means more data can flow each second, so the same job finishes sooner. A family uploading 2 GB of holiday photos to cloud storage is shifting about 16,000 megabits, which takes roughly 16 seconds on a 1000 Mbps connection but around 200 seconds on an 80 Mbps line.

Bandwidth is not the speed at which an individual bit travels along a cable or through the air, since signals move at roughly the same speed whatever the bandwidth. What changes is how many bits can be sent in each second, so a higher bandwidth means a large transfer finishes sooner and network performance generally improves.

Bandwidth is also different from latency: latency is the delay before data reaches the other end, while bandwidth sets how much of it can arrive in each second.